Biochemical analyses are invaluable, routine tools in health-related fields such as immunology, pharmacology, gene therapy, and the like. In order to successfully implement therapeutic control of biological processes, it is imperative that an understanding of biological binding between various species is gained. Indeed, an understanding of biological binding between various species is important for many varied fields of science.
Many biochemical analytical methods involve immobilization of a biological binding partner of a biological molecule on a surface, exposure of the surface to a medium suspected of containing the molecule, and determination of the existence or extent of molecule coupling to the surface-immobilized binding partner.
One such technique recently introduced is surface plasmon resonance. This technique utilizes a glass slide having a first side on which is a thin metal film and a second side opposite the first side (known in the art as a sensor chip), a prism, a source of monochromatic and polarized light, a photodetector array, and an analyte channel that directs a medium suspected of containing an analyte to the exposed surface of the metal film. A face of the prism is separated from the second side of the glass slide (the side opposite the metal film) by a thin film of refractive index matching fluid. Light from the light source is directed through the prism, the film of refractive index matching fluid, and the glass slide so as to strike the metal film at an angle at which total internal reflection of the light results, and an evanescent field is therefore caused to extend from the prism into the metal film. This evanescent field can couple to an electromagnetic surface wave (a surface plasmon) at the metal film, causing surface plasmon resonance.
Coupling is achieved at a specific angle of incidence of the light with respect to the metal film (the SPR angle), at which the reflected light intensity goes through a minimum due to the resonance. This angle is determined by a photodetector array as the angle of reflectance and is highly sensitive to changes in the refractive index of a thin layer adjacent to the metal surface. Thus it is highly sensitive to coupling of an analyte to the surface of the metal film. For example, when a protein layer is adsorbed onto the metal surface from an analyte-containing medium delivered to the surface by the analyte channel, the SPR angle shifts to larger values, and this shift is measured by the photodetector array. An article by Stenberg, Persson, Roos, and Urbaniczky, entitled "Quantitative Determination of Surface Concentration of Protein with Surface Plasmon Resonance Using Radiolabeled Proteins", Journal of Colloid and Interface Science, 43: 2, 513-526 (1991), and references therein, describe the technique of surface plasmon resonance. Instrumentation for analysis via surface plasmon resonance is available from Pharmacia Biosensor, Piscataway, N.J., under the trademark BIAcore.TM..
Although the introduction of SPR represents an extremely valuable contribution to the scientific community, current state-of-the-art SPR instrumentation lacks the sensitivity needed to detect and analyze certain biological interactions that are at the forefront of scientific inquiry. Experimentation conducted in connection with the instant invention has led to identification of several complications associated with prior art sensor chips, which complications hinder the sensitivity of current SPR techniques. According to one technique for immobilization of a binding partner of an analyte on a surface plasmon resonance sensor chip, long-chain hydroxyalkyl thiols are adsorbed onto a gold surface as a monolayer, the monolayer's exposed hydroxy groups are activated with epichlorohydrin under basic conditions to form epoxides, a carboxylated dextran gel layer is covalently attached to the monolayer, and a proteinaceous binding partner of an analyte is first electrostatically adsorbed onto the dextran gel layer and then covalently attached thereto. This technique is described in an article by Lofas and Johnsson entitled, "A Novel Matrix on Gold Surfaces in Surface Plasmon Resonance Sensors for Fast and Efficient Covalent Immobilization of Ligands", J. Chem. Soc., Chem. Comm. 1526-1528 (1990).
The effectiveness of this approach is hindered by several factors. First, covalent attachment of the proteinaceous binding partner to the gel can affect the binding partner's viability, or activity. Second, covalent attachment of the binding partner to the gel can not be effected with control over the orientation of the binding partner with respect to the surface of the chip (and, importantly, with respect to an analyte-containing medium). Third, non-specific interactions at the gel are promoted by the negative charge that it carries.
According to another technique, a mixed monolayer of hydroxyl and biotin-terminated alkane thiols is prepared on a gold surface, streptavidin is bound to the surface-bound biotin, and biotin-labeled proteins, that are binding partners of analytes, then are attached to empty sites on the streptavidin. However, because biotin must be covalently attached to the protein, this approach lacks control over orientation of the binding partner with respect to the analyte medium, and inactivation of the proteinaceous binding partner due to the formation of covalent linkage can occur. This technique is described in an article by Spinke, Liley, Guder, Angermaier, and Knoll entitled, "Molecular Recognition at Self-Assembled Monolayers: The Construction of Multicomponent Multilayers", Langmuir, 9, 1821-1825 (1993).
Accordingly, a general purpose of the present invention is to provide an easily-synthesized chemical species that readily adheres to a surface, and that facilitates surface immobilization of a binding partner of a molecule desirably captured at the surface with a high degree of sensitivity and minimal to zero non-specific binding. It is another purpose of the invention to provide an article with a surface having a high degree of sensitivity for a biological molecule. Another purpose of the invention is to provide a method of capturing a biological molecule, for example at a biosensor surface, by exploiting biological binding interactions that are extremely sensitive to molecular conformation and molecular orientation.